Carbon dioxide emissions generated from the combustion of fossil fuels, such as gasoline and diesel, has been attributed to causing globe warming through the well known greenhouse effect. Alternative energy carriers, such as hydrogen, are currently under development as a replacement for the traditional fossil fuels in transportation applications due to its minimum impact on the environment. On-board hydrogen storage technology represents a critical component for transportation applications for the future H2-based economy. As such, the Department of Energy (DOE) has established targets necessary to make a hydrogen storage system technically and economically feasible by 2010. To meet the DOE's 2010 targets, the storage system must have a minimum gravimetric capacity of 0.06 kg H2/kg—ads and a volumetric capacity of 0.045 kg H2/L at ambient temperature, and the system must cost less than $4/kWh (i.e. dG≧0.06 kg H2/kg—ads, dV≧0.045 kgH2/L, Cost≧$8/kg—Abs). However, no current technology meets these goals.
At present, hydrogen storage technologies being developed include compression, metal hydrides, chemical hydrides and physisorption-based materials. Among them, physisorption-based materials, such as porous carbon, have some unique advantages. Hydrogen adsorption on an open carbon surface originates from van der Waals attraction with a low free energy of ˜3.8 kJ/mol. This weak interaction enables H2 molecules to physisorb on the adsorbent surface with high mobility, yet allows them to release easily with minimum energy input when needed. Metal and chemical hydrides, on the other hand, require significant energies to extract H2 by breaking the chemical bonds. The carbon-based materials are also, in general, lightweight and inexpensive compared with the hydrides. The weak H2 physisorption, nonetheless, also contributes to the deficiency of the carbon-based materials. The average kinetic energy of H2 at ambient temperature exceeds to that of physisorption, which limits the storage capacity of open-structured materials, such as activated carbons, even if they have very high specific surface areas. An ideal carbon-based, non-dissociative adsorbent should have H2 adsorption energy in the range of 10˜40 kJ/mol.
However, in practice, fine-tuning the physisorption energy of H2 on the carbon surface has proven very challenging. Recently, it has been demonstrated that a H2 storage capacity of up to 3 wt % can be achieved on carbon single-wall nanotubes (A. C. Dillon, K. M. Jones, T. A. Bekkedahl, C. H. Kiang, D. S. Bethune, and M. J. Heben, Nature 386, (1997)). This finding demonstrated that relatively high H2 storage can be indeed achieved through a non-dissociative processes in porous materials with narrow channel and interstitial spacing. The carbon single-walled nanotubes are fabricated via laser sublimation or arc discharge at a low yield. However, carbon single-walled nanotube are prohibitively expensive as a commercial adsorbent material.
Consequently, there is a need for a commercially feasible and highly efficient absorbent carbon based material. The commercial application of a hydrogen storage technology will require hydrogen storage materials that can be manufactured in large quantities at low cost while still possessing sufficient gravimetric and volumetric properties. One cost-effective alternative to expensive and hard to manufacture carbon single-walled carbon nanotubes is the porous polymers. Traditionally, polymers have not been viewed as viable gas adsorbents due to their densely packed intermolecular space and the lack of surface area or porosity. In 2002, it was reported in the art that a hydrogen adsorption of up to 8 wt % was achieved over acid-treated conductive polymers, such as polyaniline and polypyrrole. However, this claim was later refuted.